Transposase activity was thought to be extinct in humans because DNA movement can be deleterious in higher organisms, resulting in genomic instability and perhaps malignancy. However, we isolated a human transposase protein termed Metnase that had both a Transposase domain that functions as an endonuclease and a SET histone methylase domain. The Transposase domain has preferential endonuclease activity for supercoiled DNA, and the SET domain was able to methylate histone 3 at lysine 36, associated with open chromatin. Metnase enhances resistance to radiation therapy, and improves repair of DNA double strand breaks (DSBs) via the non-homologous end-joining pathway (NHEJ). Both the SET and Transposase domains were required for the NHEJ repair activity. Metnase was found to interact with the NBS1, an early NHEJ repair pathway component, DNA Ligase IV, a final component of the NHEJ pathway, and Pso4, an uncharacterized DSB repair component. We found that Metnase decreases the incidence of long deletions at the repaired DSB junction site. Metnase is phosphorylated at S495, and this is essential for its NHEJ repair activity. We show that Metnase decreases the rate of inter-chromosomal translocation when there are simultaneous DSBs on distinct chromosomes, consistent with its NHEJ repair activity. We also found that Metnase also mediates resistance to Topo II poisons, which cause DSBs, in cancer cell lines. Thus, Metnase is a novel component of the NHEJ repair pathway, and links histone modification to DSB repair. The mechanism by which Metnase improves DNA repair is unknown, but its SET histone methylase domain is essential to its NHEJ activity. Recent studies in yeast indicate that the modification of chromatin, including histone methylation, may be an important part of DSB repair. However, despite its potential importance, the connection between such epigenetic chromatin modifications and NHEJ DSB repair is not well defined. Using a novel ChIP assay to analyze proteins associated with a single defined DSB, we found that Metnase localized to that DSB, and dimethylated H3K36 there. We also found evidence that dimethylated H3K36 may recruit early NHEJ components, such as ATM and the MRN complex, to the DSB. Based on these data, we hypothesize that Metnase plays an important role in the epigenetic regulation of NHEJ DSB repair. This hypothesis will be explored in four aims that translate molecular mechanisms to clinical relevance- 1) What are the structures of Metnase that are essential for its histone methylase activity? 2) What are the histone alterations Metnase makes around a DSB sites? 3) What is the mechanism by which Metnase's histone methylation enhances DSB repair? 4) Can the histone methylase activity of Metnase be exploited clinically?